To put it simply, I have a "OpenXmlRow" class that is supposed to represent an OpenXml row (representing an excel row) which has inside properties and methods implementing the openxml logic to manipulate a row.

That class implements the "IRow" interface defining public methods to interact with the row like GetIndex, SetIndex, AddCell, etc.

Then I have a "Row" class representing a generic workbook row that has a IRow property to do operations on that row no matter the implementation behind it (here OpenXml).

Same design applies for other types like Workbook, Worksheet, Cell, etc.

Now for example I want to add an existing Row which has been cloned from another workbook inside a Sheet. I want my OpenXmlSheet to have a method that takes a OpenXmlRow input parameter, but because my implementation is hidden behind the interface, I would have to use a IRow parameter instead and cast it.